We assume /utf-8 for all C builds unless /source-charset or
/execution-charset is specified, but then this will cause trouble for
Visual Studio 2013 or earler since the /utf-8 flag is only supported
since Visual Studio 2015. Specifically, if we try to check whether
compiler flags are supported, those checks will fail since /utf-8 is
never supported on these older Visual Studio versions.
Drop /utf-8 from get_always_args() if we are using Visual Studio 2013
or earlier.
9bbceeee06